The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Modern motor vehicle automatic transmissions, especially those for passenger cars, sport utility vehicles and light trucks typically include a plurality of interconnected planetary gear assemblies which cooperate with friction clutches and brakes to provide a sequence of forward gears and reverse. Just as typically, the clutches and brakes are controlled by hydraulic actuators or operators controlled by the outputs of a plurality of spool valves in a valve body which are, in turn, controlled by solenoids under the control of a transmission control module (TCM).
The transmission generally operates in one of a plurality of modes of operation including out-of-Park driving modes and a Park mode. The out-of-Park driving modes include forward gears or speed ratios, i.e., Drive mode, at least one reverse gear or speed ratio, i.e., a Reverse mode, and a Neutral mode. Selection of the various driving modes is typically accomplished by engaging a shift lever or other driver interface device that is connected by a shifting cable or other mechanical connection to the transmission. Alternatively, the selection of a driving mode may be controlled by an electronic transmission range selection (ETRS) system, also known as a “shift by wire” system. In an ETRS system, selection of the driving modes is accomplished through electronic signals communicated between the driver interface device, the transmission control module and the transmission. The ETRS system reduces mechanical components, increases instrument panel space, enhances styling options, and eliminates the possibility of shifting cable misalignment with mechanical transmission range selection components.
While previous ETRS subsystems are useful for their intended purpose, there is a need for new and improved ETRS hydraulic control system configurations within transmissions which exhibit improved performance, especially during transmission default conditions. A default condition is a hydraulic state that the transmission experiences in the absence of electronic control. A transmission in default no longer has the capability to electronically command solenoids to achieve the desired gear state. The default condition may have been intentionally commanded (e.g. when diagnostics indicate corrupt solenoid drivers, corrupt controllers, controller shutdown at high temperatures) or can occur unintentionally due to a hardware failure (e.g. controller failure, wiring harness failure, solenoid driver failure). For some transmission configurations, the hydraulic control system shifts the transmission to neutral or park during a default condition. While a shift to neutral of park is preferable to random, uncontrolled operation of the transmission, there is a need for an improved ETRS hydraulic control system for use in a hydraulically actuated automatic transmission that can provide multiple drive states during a default condition.